More Exercise or Need Tummy Tuck/skin Tightening? (photo)

4 months post partum after 3 pregnancies and have been working REALLY REALLY hard with diet and working out. I am not happy with my skin tone on my stomach and am wondering if I just "keep at it" if it will go away over time and tighten up OR if I need to do something like skin tightening procedures to ever get that flat/not floppy tummy. Thanks for your help! (I am 5'8 and 140 lbs at the moment)

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